Added UserEntityInterface

src/Entities/Interfaces/UserEntityInterface.php

@@ -0,0 +1,12 @@
+<?php
+
+namespace League\OAuth2\Server\Entities\Interfaces;
+
+interface UserEntityInterface
+{
+    /**
+     * Return the user's identifier
+     * @return mixed
+     */
+    public function getIdentifier();
+}